1. Field of the Invention
The present invention relates to a cable transmission system, and more particularly, to a QAM (Quadrature Amplitude Modulation) symbol mapping method and apparatus for a 64 QAM mode and a 256 QAM mode in downstream transmission.
2. Discussion of Related Art
In a cable transmission system, downstream transmission is performed according to ITU-T Recommendations J. 83, Annex B. FIG. 1 is a block diagram illustrating a conventional process of processing a downstream signal. In a process for a downstream modulation, a Moving Picture Experts Group (MPEG)-2 data stream input on a per-packet basis is framed in an MPEG frame unit 110, and then a forward error correction algorithm is applied in a Forward Error Correction (FEC) encoder 120, thereby obtaining reliable data corresponding to a channel. A FEC code output from the FEC encoder 120 is QAM-modulated through a QAM modulator 130 and transmitted as a Radio Frequency (RF) signal through a cable channel 140. Downstream demodulation is performed through a QAM demodulator 150, a FEC decoder 160, and an MPEG frame unit 170 in a process that is the reverse of modulation. The MPEG framing process provides a parity check pattern for packet synchronization between a transmitter and a receiver, and the QAM modulation process supports a 64 QAM mode and a 256 QAM mode. In the FEC encoding process, a concatenated coding method is used to correct error, which uses a Reed-Solomon code having ‘t’ number of error correction abilities as an outer code and uses a Trellis Coded Modulation (TCM) code generating a coded modulation code as an inner code. According to this method, an error that is not corrected in an inner decoder can be corrected in an outer decoder, so that an error rate becomes virtually zero.
FIG. 2 is a detailed block diagram illustrating a procedure of FEC processing. As shown in FIG. 2, a FEC encoder includes a Reed-Solomon encoder 210, an interleaver 220, a randomizing unit 230, and a trellis encoder 240. A FEC decoder includes a trellis decoder 260, an inverse randomizing unit 270, a deinterleaver 280, and a Reed-Solomon decoder 290. The Reed-Solomon encoder 210 codes MPEG transport streams 128 and 122 using a RS block code. The RS block code is comprised of 128 symbols per block. Among them, only 122 symbols are information symbols, and six symbols are parities for error correction. Therefore, the RS block code performs error correction for up to a maximum of three symbols. The RS block code is used identically in the 64 and 256 QAM modes. The interleaver 220 is used to efficiently cope with an erroneous symbol (cluster error, burst error) generated in channel transmission. A convolutional interleaver structure is programmable in the 64 and 256 QAM modes and supports various interleaving modes. The randomizing unit 230 prevents the interleaved data from having a specific pattern by randomizing it, prevents an RF modulated signal from being mixed with signals of other channels, and enables extraction of synchronization at a receiver. The randomizing unit 230 generates a pseudo noise code predefined at the receiver, and adds input data to the generated pseudo noise code to output randomized data.
The trellis encoder 240 performs a TCM, which is a channel coding method for obtaining a high coding gain from a bandwidth-limited channel, and is embodied in a combined format of a coding technology and a modulation technology. The TCM structure is comprised of a 64/256 QAM modulator and a convolutional encoder having a limited state.
FIG. 3 is a block diagram illustrating a detailed structure of a conventional trellis encoder. As shown in FIG. 3, the trellis encoder separately includes a 64 QAM modulation system 300a and a 256 QAM modulation system 300b, and selects a suitable one of the two modulation systems according to a channel environment. Here, a symbol is mapped in each modulation mode using various differential coding methods, and accordingly, an input bit should be generated to have a phase and amplitude of the corresponding symbol. The 64 QAM modulation system 300a includes a data formatting unit 310a for receiving RS symbol bits from the randomizing unit and classifyiing the received RS symbol bits into group bits according to a predetermined rule; a coder 320a for receiving predetermined group bits from the data formatting unit 310a and coding the received group bits; and symbol mapping units 330a for receiving the non-coded group bits output from the data formatting unit 310a and the coded group bits output from the coding unit 320a and performing symbol mapping. In detail, the coder 320a includes a differential coder 322a and a Binary Convolution Coder (BCC). The 256 QAM modulation system 300b has substantially the same construction as the 64 QAM modulation system 300a. 
Korean Patent Publication No. 2000-14483 entitled “QAM MAPPER AND DEMAPPER FOR DOWNSTREAM TRANSMISSION OF CABLE MODEM” discloses a QAM mapping apparatus in which, in order to reduce a size of a memory used in a symbol mapping lookup table, phase information of a QAM symbol is determined by mapping two convolution coded bits of a QAM-TCM modulation mode to any one of four phases according to a method for differential coding between QAM symbols continuous on a time axis, and amplitude information uses a remaining bit pair as is.
However, the QAM mapping apparatus has a drawback in that since the method for extracting the phase information of the QAM symbol differs depending on the differential coding methods, a hardware structure should be changed to match differential coding mechanisms. Further, since the QAM mapping apparatus also uses memory and therefore its operation speed is limited by memory access time, it is not suitable for a high-speed data transfer system.
Accordingly, it is required to develop a QAM symbol mapping apparatus that is capable of extracting the phase information of the QAM symbol irrespective of differential coding mechanisms and does not use a memory so that it can be suitably applied to a high-speed data transfer system.